a) Field of the Invention
The invention is directed to an arrangement for generating extreme ultraviolet radiation from a plasma generated by an energy beam with high conversion efficiency in which a pulsed energy beam is directed in a plasma generation chamber to a location where it interacts with a target, a target feed device contains a mixing chamber for generating a mixture of particles of an emission-efficient target material with at least one carrier gas and an injection unit for dispensing individually defined target volumes into the plasma generation chamber in a metered manner in order to supply only as much emission-efficient target material to the interaction location as can be converted into radiation by an energy pulse. The invention is applied in particular in radiation sources for EUV lithography for the fabrication of semiconductor chips.
b) Description of the Related Art
Known “clean fuels” (target materials such as xenon) are not sufficiently efficient for the generation of EUV radiation based on a plasma which is excited by a pulsed energy beam for emitting in the EUV spectral band around 13.5 nm because their conversion efficiency (ratio of the emitted energy in the desired EUV spectral band to the (laser) excitation energy) is only about 1%. By “clean fuel” is meant that it does not produce a “coating” of components of the radiation source, i.e., it does not generate precipitation (contamination) on surfaces (particularly optical surfaces). Metallic target materials (e.g., elements of groups IV to VII of the 5th period of the periodic table of elements) are substantially more efficient for generating EUV at 13.5 nm (e.g., tin has a conversion factor of approximately 3%), but produce a “coating”, i.e., in exciting plasma they generate debris which results especially in precipitation but also leads to ablation of components of the radiation source, especially optical components. Further, ablation processes (removal of material from optical surfaces) which are caused by the high kinetic energy of unconsumed target particles not converted into luminous plasma are appreciably reduced for “clean fuels” (e.g., xenon) compared to metallic target materials.
Pure tin (Sn) delivers a broad-band spectrum around 13.5 nm±2% (desired EUV spectral band for semiconductor lithography, so-called EUV in-band radiation) but also has significant proportions outside the desired EUV spectral band for semiconductor lithography (EUV out-of-band radiation). These out-of-band radiation components are undesirable because they contribute to unnecessary heating of the optics and other source components.
In order to make use of metal-containing targets, it was known in the prior art to use metallic solutions at room temperature as target droplets for laser-generated punctiform plasma. In U.S. Pat. No. 6,831,963 B2, copper compounds and zinc compounds in particular such as chloride solutions, bromide solutions, sulfate solutions, nitrate solutions and organometallic solutions are described as metallic solutions which can be applied in the vicinity of optical components without damage to the latter because hardly any debris is produced. However, substantially only radiation in the range from 11.7 nm to 13 nm is generated, which must be classified as out-of-band radiation components within the meaning of the above-stated requirements of EUV lithography. The same situation is also described for tin compounds, particularly tin chloride, in US 2004/0208286 A1.
As is disclosed in WO 2002/046839 A, an injection of droplets in liquids (e.g., tin as compound or nanoparticle) makes it possible to limit the amount of convertible target material. However, it is disadvantageous that all of the carrier liquids or solvents known for this purpose contain component parts which are damaging to optics (carbon coating, oxygen oxidation, etc.).
WO 2004/056158 A2 describes a device for generating x-ray radiation and EUV radiation in which a mist with an atomic density of >108 atoms/cm3 is generated for increasing the target density of the smallest possible droplets (on the order of the laser wavelength). The improved target density is generated by the absorption of the target liquid in a nonreactive gas in that an electro-magnetically switchable valve is connected to an ultrasonic nozzle via an expansion duct which is outfitted with heating means for increasing temperature in order to generate a supersaturated vapor and supply it by bursts through the target nozzle for generating plasma. The disadvantage here consists in the elaborate metering procedure and in that the target density drops off quickly after exiting the target nozzle.
Gaseous injections of nanoparticles into a carrier gas, as is described in EP 0 858 249 B1 and WO 2004/084592 A2, are generally not sufficiently concentrated because the particle-containing “gas cloud” expands rather quickly so that the density is too low for an efficient excitation, e.g., by means of a laser, even at a short distance from the injection site (on the order of 1 cm). Therefore, the excitation must be carried out in the vicinity of the injection opening, and limiting the particle quantity to the amount needed for complete energy conversion cannot be accomplished in a simple manner.
WO 2004/084592 A2 discloses a possibility for metering solid target material. A chamber system is provided in which a mixing of solid or liquid target clusters in a gas is carried out in a first chamber. As a result, a “focused mass flow” is generated in a second chamber and arrives in the third chamber for plasma generation through a periodically opening shutter device as a pulsed mass flow in order to provide the necessary amount of convertible target material for each laser pulse and accordingly to reduce the proportion of unconverted target material in the plasma chamber. The target material that is blocked in the second chamber by the shutter device is sucked out and can be reused.